remove use of dxguid.lib

That library is no longer included in recent SDKs (Vista, 7.x, 8.x).

This method of including InitGuid.h (once only) appears to be the current approach.

It might also work when using earlier SDKs but I'm not sure.
Merge pull request #19 from neogeographica/no_dxguid_lib

remove use of dxguid.lib